Polymer seaming with diffusion welds

ABSTRACT

The present invention is a method for seaming a polymeric material. The method involves forming an interface between a plurality of separate polymeric sheets of materials. Next, heat and pressure are applied to facilitate the diffusion of the polymer molecules at the interface. A diffusion weld is made when the polymer molecules diffuse across the interface.

This application is a continuation in part of, and claims priority to,pending U.S. patent application Ser. No. 11/740,267, which was filed onApr. 25, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the manufacture of polymer films.More specifically, the invention relates to polymer films that have beenseamed and a method of seaming sheets of polymeric material togetherwith diffusion welding.

2. Background Art

Polymer films are frequently produced in a rolled form. Such rolls areoften seamed together to combine multiple rolls, to make a longer roll,or to make a larger polymeric sheet structure. The films can be thin,such as 2 mils thick or less, but the films can also be thicker. Thinsheets are often sensitive to mechanical tears and contamination. Somematerials are not as strong as others, so thin sheets are even moreprone to mechanical tears, rips, or other damages.

Thermoset plastics are polymeric materials that irreversibly cure. Thecure may result from heat, irradiation, or a reaction. The reaction canbe between two different compounds, such as with an epoxy. Some polymersare formed with two different types of monomers, such as a polyimide.Most polyimides are considered an AA-BB type polymer because twodifferent classes of monomers are used to produce the polyimide polymer.One class of monomer is called an acid monomer, and is usually in theform of a dianhydride. The other type of monomer is usually a diamine,or a diamino monomer.

Thermoset plastics often have no melting point. The thermoset plasticmay decompose before melting, or the thermoset plastic may begindecomposing as it approaches its melting point. Thermoset plastics willgenerally have a glass transition temperature, where the plastic becomesless brittle and softer above the glass transition temperature. Becausethermoset plastics do not have a true melting point, or they begin todecompose as they approach their melting point, melt welding is noteffective in joining separate pieces. Other techniques are needed tojoin sheets of thermoset plastic together. Thermoplastic materials, onthe other hand, can be melted without significant decomposition. Sheetsof thermoplastic materials can be overlapped and melted together, whichis referred to as melt welding. Thermoplastic materials do have a glasstransition temperature which is lower than its melting point. Polymericmaterials can be thermoplastic or thermoset.

Polyimides are one type of thermoset plastic with many desirablecharacteristics. Other thermoset plastics include polybenzoxazoles andepoxies. There are also examples of polyimides, polybenzoxazoles, andepoxies which are thermoplastic. Polyimide sheets can be used for spaceapplications where weight is an important factor. Thinner sheets weighless, but still need to be strong enough to function. Also, thetemperature in space can change significantly and rapidly. If twodifferent materials are joined together, and the different materialshave different coefficients of thermal expansion (CTEs), the rapid andlarge temperature changes can cause the materials to separate. Anadhesive and a polymer will often have different CTEs.

Thin sheets of polyimides are commonly used on satellites and otherspace flight equipment. Thin sheets are also referred to as films inthis disclosure. They are typically thin polymer sheets which may havean applied reflective metallized evaporative coating. These sheets areused as solar shields, solar concentrators, solar sails, etc. Areflective membrane may be used to protect orbital structures andequipment such as satellites from direct exposure to solar radiant flux.Alternatively, a reflective membrane may be used to concentrate solarenergy on equipment such as a solar panel that powers a satellite. Othercoatings can also be used for various applications.

Prior art techniques of seaming include bonding the materials with anadhesive or alternatively employing a mechanical fastener. However, theapplication of the adhesive seaming chemicals is challenging and proneto contamination. Additionally, commonly used adhesives, such asacrylic-based or silicone-based, lose mechanical strength and increasein stiffness at temperatures below their glass transition temperature(Tg), such as temperatures encountered in space. Differences in the CTEbetween the polymer and the adhesive can also lead to premature seamfailure. A mechanical fastener, such as stitching with thread, is proneto tearing especially if single layers of materials are seamed together.Consequently, a method for effectively seaming thin polymer sheets isneeded.

SUMMARY OF THE INVENTION

In some aspects, the invention relates to a plurality of thermosetplastic sheets diffusion welded together. In other aspects, theinvention relates to a method for seaming a polymeric material usingdiffusion welding. In other aspects, the invention relates to a methodfor seaming a polymeric material using diffusion welding, where thepolymeric material has a coating. The diffusion welding involves formingan overlapping interface of the polymeric material, and applying heatand pressure to the interface to form a diffusion weld between thematerials.

BRIEF DESCRIPTION OF THE DRAWINGS

It should be noted that identical features in different drawings areshown with the same reference numeral.

FIG. 1 shows a thermal seaming method that utilizes a heated stylus inaccordance with one embodiment of the present invention.

FIG. 2 shows a thermal seaming method that utilizes a heated roller inaccordance with one embodiment of the present invention.

FIG. 3 shows a thermal seaming method that utilizes a heated plunger inaccordance with one embodiment of the present invention.

FIG. 4 shows a thermal seaming method that utilizes a heated plungerwith a local tensioner.

FIG. 5 shows polymeric sheets having a core and a coating, where thepolymeric sheets are positioned to be diffusion welded.

FIG. 6 shows a composite thermoset plastic sheet with a seam.

DETAILED DESCRIPTION The Diffusion Weld

A method for seaming thin film polymers has been developed. The methodis effective for at least some thermoset polymers which are notconducive to melt welding. Thermoset polymers which can be seamedinclude at least some polyimides and some polybenzoxazoles. Examples ofthin film polyimides include, but are not specifically limited to,PMDA/ODA (KAPTON®), and 6FDA/4BDAF (CP1), where PMDA stands forpyromellitic dianhydride, ODA stands for oxydianiline, 6FDA stands for4,4′-(hexafluoroisopropylidene)diphthalic anhydride, and 4BDAF standsfor 2,2-bis[4-(4aminophenoxy)phenyl] hexafluoropropane.

The method includes the application of heat and pressure to thermallyweld two layers of material together. The method produces a weld calleda “diffusion weld” that is acceptably strong and effective under a widerange of temperature conditions. The method is effective in seamingcertain thin polymeric materials that may have a thickness of less than2 mils, or even thinner materials with a thickness of less than 5microns. The method is most beneficial for thermoset polymers becausemelt welding is not a viable option, but the method is also effective oncertain thermoplastic materials. The method can also be effective forthicker polymeric materials.

“Diffusion welding” involves joining polymers by applying a necessaryamount of heat and pressure for a defined period of time. This resultsin a plastic flow of material at the interface between the two bodies ofmaterials. The plastic flow removes, agglomerates, or buries surfacecontaminants, flattens surface asperities, and brings the bodies intointimate contact. The molecules of the separate pieces of polymericmaterial are diffused and repositioned across the interface of thebodies until the interface becomes indistinct and energy is required toseparate the bodies. Some polymeric molecules will become repositionedsuch that the molecules are present in more than one polymeric piece, sothe molecules serve to bind the separate pieces together.

In order for diffusion welding to occur, the edges of the polymericsheet to be seamed (called a “faying surface”) must be overlapped andheated to a suitable bonding temperature. The overlapped faying surfacesare referred to as an interface. Many polymeric materials have adistinct glass transition temperature (T_(g)) where the material softensand distorts. The suitable bonding temperature for diffusion welding canbe within about 15 degrees centigrade below the T_(g) to about 15degrees centigrade above the T_(g). It is also possible to applytemperatures more than 15 degrees centigrade above the T_(g) for shorterperiods of time, so the time of application and the temperature shouldbe considered together when forming a diffusion weld. There may also beinstances where a temperature of less than 15 degrees centigrade belowthe Tg can be used with the proper application time. Once heated,pressure is added to the interface in order to promote diffusion of thesoftened material across the interface of the bodies. It is alsopossible to apply pressure first, and then temperature, or to applypressure and temperature at the same time. The exact temperature needed,the amount of pressure needed, and the application times for thetemperature and pressure are determined by experimentation for eachdifferent application. Factors which can affect the temperature,pressure, and bonding time include the materials being diffusion welded,the thickness of the materials and/or coatings, the types of coatings,and other process conditions.

One advantage of the present invention is that a seam created by thedisclosed invention exhibits decreased tear propagation at the seampoints. Another advantage is that the present invention eliminates theneed for adhesives to join two materials. This prevents thermaldeformations due to inherent CTE differences of the materials and theadhesive. These thermal deformations can present challenges attemperatures below the T_(g) of adhesives when the modulus of theadhesives tends to increase. Another advantage is that characteristicconductivity of conductive materials, such as carbon-filled polyimide,is retained at the seaming point.

Methods of Producing the Diffusion Weld

In one embodiment of the present invention, the polymeric materials areheated by contact with a heating tool that also applies pressure to theinterface. FIG. 1 shows one example of a thermal seaming system 10 thatuses a heated stylus 16. A first sheet of a polymeric material 12 a anda second sheet of polymeric material 12 b are positioned with theirrespective faying surfaces forming an overlapping interface 14. Theoverlapped interface 14 can be about one inch across, but wider overlapsallow more area for a larger diffusion weld, and smaller overlaps canalso be used. The interface 14 is placed on an insulative backing 18 anda protective overlay 20 can be placed over the top of the interface 14.The heated stylus 16 is then dragged 24 across the overlay 20. Thestylus 16 applies both heat and pressure to the interface 14 and forms acontinuous diffusion weld 22.

FIG. 2 shows an alternative thermal seaming system embodiment 30 of thepresent invention that utilizes a heated wheel 32. In this embodiment, aheated wheel 32 is rolled 34 across the interface 14 to apply both heatand pressure that forms a continuous diffusion weld 22. While thisembodiment is shown without a protective overlay, it should beunderstood that embodiments exist for diffusion welding with or withouta protective overlay. The need for a protective overlay is determined byexperimentation based on several factors, including the materials beingdiffusion welded, the thickness of the materials, the type of apparatusapplying the heat and pressure, the diffusion welding temperature andpressure used, and the application time for the temperature and pressureapplication.

FIG. 3 shows yet another thermal seaming system embodiment 40 of thepresent invention that utilizes a plurality of heated plungers 42.Alternate embodiments could use a single heated plunger 42. In thisembodiment, the heated plungers 42 move intermittently in vertical 44and horizontal 46 directions applying heat and pressure at variouspoints along the interface 14. The effect is to create multiple “spot”diffusion welds 48. The plungers 42 apply heat and pressure to producespot diffusion welds 48, and then move vertically 44 out of contact withthe interface 14. The plungers 42 then move horizontally 46 tore-position themselves for the next spot diffusion welds 48. Then theplungers 42 move vertically 44 downward into contact with the interface14, and produce the next spot diffusion welds 48 by applying heat andpressure. Reference to the plunger 42 moving horizontally or verticallymeans the plunger motion includes a horizontal or vertical component, sothe plunger 42 can move both horizontally and vertically at the sametime. Vertical motion is used to contact or separate the plunger 42 fromthe interface, and horizontal motion is used to re-position the plunger42 for another spot diffusion weld 48.

The plungers 42 can be used to produce spot diffusion welds 48, insteadof the continuous diffusion welds discussed above. The plungers 42 couldalso be dragged to produce continuous diffusion welds, or a mixture ofspot and continuous diffusion welds, as desired. The plungers 42 canprovide simpler temperature control mechanisms, because the diffusionwelding process is intermittent and focused at the end of the plunger42. Also, the contact end of the plunger 42 can have a small area, so ahigh pressure can be applied at the spot of the diffusion weld. However,the spot diffusion welds 48 may not produce a complete seal between thefirst and second polymeric sheets 12 a, 12 b. Each spot diffusion weld48 may be isolated from every other spot diffusion weld 48, meaning eachspot diffusion weld 48 does not contact any other spot diffusion weld48. When the spot diffusion welds 48 are isolated, a complete seal maynot be formed between the first and second polymeric sheets 12 a, 12 b,

Most welds of materials with a non-zero CTE have differential shrinkagedistortion of the joined bodies upon cooling. This is the result oftemperature gradients which are characteristic in practically allwelding techniques. This distortion is undesirable for many applicationsusing diffusion welded thin film membranes. It can be reduced by locallytensioning the sheets of polymeric material 12 a, 12 b in the vicinityof the diffusion weld during the welding process. When the tension isreleased, the local residual compression stress tends to counteract thelocal residual tensile stresses that result from the weld. FIG. 4 showsone embodiment of the thermal seaming system 50 that utilizes a heatedplunger 42 in conjunction with a local tensioner 52. In this embodiment,the tensioner 52 is circular-shaped. The plunger 42 contacts theinterface 14 through a tensioner opening 54 to create the diffusion weld48.

The examples shown in FIGS. 1-4 demonstrate diffusion welding of shearor overlapping joints. Other embodiments of the present invention couldbe used to produce butt or tensile joints. In these embodiments, thefaying surfaces are abutted rather than overlapped, so the polymericsheets are positioned edge to edge. A separate seaming sheet can then bepositioned over the abutted edges of the polymeric sheets, so there isan overlapped interface. The heated bonding tool is brought in contactwith the overlapped interface to form a diffusion weld.

While these embodiments show a single device that provides both heat andpressure to the interface, it should be understood that alternativeembodiments could use multiple devices to provide these as desired.Further, the temperatures and pressures applied will depend on thespecific characteristics of each material. However, it is common for thesuitable bonding temperature to exceed 500 F yet still not reach themelting point of the material. If two dissimilar materials, such as apolyimide and non-polyimide, are being bonded then the material with thelower glass transition temperature (T_(g)) and/or melting point shouldbe heated to a lesser temperature. One way this may be accomplished isby placing a heat source directly on the material with the higher T_(g)while placing the lower T_(g) material underneath the higher T_(g)material to avoid direct contact with the heat source. The material withthe higher T_(g) serves to insulate the material with the lower T_(g) tosome extent.

Polymeric Materials

While the present invention has been described for diffusion welding twoor more similar thermoset plastic materials, the methods disclosed mayalso be effective to diffusion weld two or more thermoplastic plasticmaterials or even to diffusion weld two or more dissimilar materials.For example, the various embodiments could be used to connect apolyimide with a different polyimide material such as diffusion weldingthe polyimide associated with the trademark KAPTON® and CP1 together.Other embodiments may involve seaming a thermoset plastic to any of thefollowing polymers: polyamides (such as polymer associated with thetrade name NYLON); polyesters such as poly(ethylene naphthalate) (PEN)or poly(ethylene terephthalate) (PET/polymers known by the trade nameMYLAR); polyamide imide (PAI/polymers known by the trade name TORLON);polyether ketone (PEK); polyether ketone ketone (PEKK); polyether etherketone (PEEK); polyether imide (PEI); polyphenylene sulfide (PPS);polyether sulfone (PES); polytetrafluoroethylene (PTFE); andpolyphenylene (PARMAX). These materials are given as examples ofalternative materials that may be diffusion welded to a thermosetplastic or to a thermoplastic plastic. It should be understood thatother materials known in the art with similar performancecharacteristics could be used as well.

It is possible to diffusion weld thermoset materials with thermoplasticmaterials if the characteristics of the different polymers areacceptable. For example, some polyimides associated with the trademarkKAPTON® have a T_(g) higher than 300 degrees Celsius, and these KAPTON®polyimides can be diffusion welded to other materials with a meltingpoint lower than the T_(g) of KAPTON®. It is also possible to diffusionweld certain thermoplastic materials together. This can include separatesheets of one type of thermoplastic polymer, or separate sheets ofdifferent types of thermoplastic polymers.

Many polymeric sheets will include coatings of one type or another, andthese coatings can affect diffusion welds. A polymeric sheet with acoating includes at least a core 60 and a coating 62, as seen in FIG. 5,where the core 60 is the base layer and the coating 62 is applied to thecore 60. Coatings 62 can be on one side of a polymeric sheet 12 a, orthey can be on both sides of a polymeric sheet 12 a. There can also bemultiple coatings 62 on a polymeric sheet 12 a. A polymeric sheet 12 acan have a first type of coating 62 on a first side 64, and a differenttype of coating on the second side 66. Each side 64, 66 can have zero,one, two, or more coatings. The coatings can change the properties ofthe polymeric sheet 12 a, so different coatings 62 and cores 60 are usedfor different applications.

Some coatings 62 can be at the interface 14 and not interfere with thediffusion weld, but other coatings can prevent or weaken the diffusionweld if present at the interface 14. For example, metallic coatingsshould not be between the layers in the interface 14 where the diffusionweld is produced. The metallic coating can be on a polymeric sheet 12 aon a side facing away from another layer, so the metallic coating is notbetween layers in the interface 14. The coating 62 on polymeric sheet 12a is facing away from polymeric sheet 12 b, and so the coating 62 onpolymeric sheet 12 a is not between layers in the interface 14. However,polyimide cores 60 with a polytetrafluoroethylene (PTFE) coating 62 oranother fluorinated polymer, such as fluorinated ethylene propylene(FEP) and perfluoroalkoxy (PFA) can be diffusion welded with the coating62 between the layers in the interface 14.

Polymeric sheets with metallic coatings cannot be diffusion welded ifthe metallic coating is between the polymers of the different sheets.One way to diffusion weld polymeric sheets with metallic coatings is toremove the metallic coating at the faying surface, so there is nocoating in the overlapped interface. Alternatively, the polymeric sheetscan be positioned with the faying surfaces abutted instead ofoverlapped, so the sheets are edge to edge. A separate seaming sheet canbe positioned over the abutting edges on the side of the polymericsheets which do not have a metallic coating, and the diffusion weld canbe made in the interface. Other alternatives include coating thepolymeric sheets after they have been seamed, or preventing a fayingsurface from being coated when the rest of the polymeric sheet receivesthe metallic coating.

Most experiments completed to date have involved diffusion welding twoseparate sheets of the same polymeric material. Diffusion welds havebeen found effective for certain materials, but more experimentation isneeded to determine what materials and what mix of materials can bediffusion welded. Below is a list of materials for which the diffusionwelding described above has been shown to work.

1. CP1 polyimide diffusion welded to CP1 polyimide;

2. KAPTON® polyimide diffusion welded to KAPTON® polyimide;

3. PTFE coated KAPTON® diffusion welded to PTFE coated KAPTON®;

4. Liquid crystalline polymer (LCP) polyester diffusion welded to LCPpolyester;

5. CP1 filled with PTFE diffusion welded to CP1 filled with PTFE;

6. CP1 polyimide diffusion welded to KAPTON® polyimide;

7. PTFE-coated KAPTON® diffusion welded to uncoated KAPTON® polyimide;

8. FEP-coated KAPTON® diffusion welded to uncoated KAPTON® polyimide;

9. PFA-coated KAPTON® diffusion welded to uncoated KAPTON® polyimide;

10. LCP polyester diffusion welded to uncoated KAPTON® polyimide;

Composite Thermoset Plastic Sheets

Thermoset plastics seamed together with diffusion welds can be producedwith the techniques described above, as shown in FIG. 6. This wouldinclude a plurality of component thermoset plastic sheets, such as afirst component thermoset plastic sheet 70 and a second componentthermoset plastic sheet 72. A plurality of component thermoset plasticsheets 70, 72 can be diffusion welded together to form one largercomposite thermoset plastic sheet 74. The composite thermoplastic sheet74 can be recognized because there is no adhesive between theoverlapping layers in the interface 14. The thermoset plastic of onecomponent sheet 70 is directly bonded to the thermoset plastic ofanother component sheet 72. If melt welding had been attempted, weldpoints would show increased levels of decomposition. The use ofstitching or other mechanical connectors is not needed, but could beused with diffusion welding if desired.

The composite thermoset plastic sheet 74 includes a seam 76 withoverlapping layers of thermoset plastic. The seam 76 may not include anyadhesive, but adhesive could also be added if desired. A seam 76 differsfrom an interface 14 in that a seam 76 includes a connection between atleast two separate sheets, and an interface 14 is an area withoverlapping layers either before or after the layers are connected. Atleast one of the component thermoset plastic sheets, such as the firstcomponent thermoset plastic sheet 70, may comprise a polyimide, but thethermoset plastic may also comprise polybenzoxazoles, polyimide filledpolytetrafluoroethylene, other thermoset polymers, or combinations ofthe above.

A component thermoset plastic sheet 70 and/or 72 may also comprise acoating 62. Reference is now made to the first component thermosetplastic sheet 70 having the coating 62, but it is to be understood thesecond component thermoset plastic sheet 72 may have a coating 62 eitherinstead of the first component thermoset plastic sheet 70, or as well asthe first component thermoset plastic sheet 70. The coating 62 on thefirst component thermoset plastic sheet 70 can be in direct contact withthe second component thermoset plastic sheet 72 at the seam 76. Thecoating may comprise polytetrafluoroethylene, polyimides,polybenzoxazoles, or other polymers. Other coatings 62, such as metalliccoatings or certain other inorganic coatings, may be on the compositethermoset plastic sheet 74 but not present between the layers of theseam 76. The coating 62 shown on the second component thermoset plasticsheet 72 is not present between the layers of the seam 76. As such, acoating 62 not present between the layers of the seam 76 would notdirectly contact another component thermoset plastic sheet 74.

The composite thermoset plastic sheet 74 may be no more than 2 milsthick, and may even be less than 5 microns thick. It is also possiblefor the composite thermoset plastic sheet 74 to have a thickness greaterthan 2 mils. Reference to the thickness of the composite thermosetplastic sheet thickness refers to the thickness in the body of thecomposite thermoset plastic sheet 74, and not the thickness at the seam76.

In one embodiment, the seam 76 is made from overlapping different edgesof one component thermoset plastic sheet, such that the compositethermoset plastic sheet 74 forms a loop or belt. In this embodiment, thefirst component thermoset plastic sheet 70 and the second componentthermoset plastic sheet 72 are represented by different edges of onesingle component thermoset plastic sheet. Alternatively, more than onecomponent thermoset plastic sheets can be connected to form a loop orbelt. Many other shapes can also be made by connecting different edgesof component thermoset plastic sheets, such as a cube or a pyramid.

A composite plastic sheet can be formed between a component thermosetplastic sheet and a component thermoplastic plastic sheet. The compositeplastic sheet therefore could include a component thermoset plasticsheet and a component thermoplastic plastic sheet diffusion weldedtogether at the seam. Coatings can be applied similar to the componentthermoset plastic sheets diffusion welded together to form a compositethermoset plastic sheet 74.

WORKING EXAMPLES

Specific working examples are provided below to further clarify specificembodiments of the current invention.

Example 1

Spot diffusion welds were produced in an overlapping seam between a 2mil thick polyimide film associated with the trademark KAPTON HN andanother 2 mil thick polyimide film associated with the trademark KAPTONHN. The overlapping seam was held in place by magnets placed above andbelow the seam such that the magnets were drawn towards each other. Abacking was positioned under the seam such that the plunger presses theseam directly into the backing. The backing was formed of a 1/1000 inchthick KAPTON tape secured with a silicon pressure sensitive adhesive toa 1/16 inch thick ceramic wick. The 1/16 inch ceramic wick waspositioned over a ⅛ inch thick nylon sheet, which was positioned over arubber foam.

A plurality of heated plungers was pressed into the polyimide materialat the seam, such that the polyimide seam was positioned directlybetween the heated plungers and the backing. The heated plungers had atip with a 0.145 inch diameter circle, and the tip was heated to 650degrees centigrade. The heated tip of the plunger was applied to theseam with a weight of 390 grams per tip for a total application time of5 seconds. The heated plungers were then retracted from the seam, andrepositioned for another spot weld. The process was repeated until thetwo polyimide sheets were thermally welded together.

Example 2

Spot diffusion welds were produced in an overlapping seam between a 2mil thick polyimide film associated with the trademark KAPTON E andanother 2 mil thick polyimide film associated with the trademark KAPTONE. The same process and parameters as described for Example 1 was usedin Example 2.

Example 3

Spot diffusion welds were produced in an overlapping seam between a 2mil thick polyimide film associated with the trademark KAPTON HN and a 2mil thick polyimide film associated with the trademark KAPTON E. Thesame process and parameters as described for Example 1 was used inExample 3.

Example 4

Spot diffusion welds were produced in an overlapping seam between a 2mil thick polyimide film associated with the trademark KAPTON HN and a 1mil thick polyimide film associated with the trademark KAPTON HN. Thesame process and described for Example 1 was used in Example 4, exceptthe plunger tip temperature was 630 degrees centigrade, the weight pertip was 300 grams, and the application time was 4 seconds.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed here.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A composite thermoset plastic sheet comprising: a plurality ofcomponent thermoset plastic sheets, the component thermoset plasticsheets including thermoset plastic molecules, and a seam connecting thecomponent thermoset plastic sheets, where the seam includes diffusionwelds comprising thermoset plastic molecules diffused across differentcomponent thermoset plastic sheets such that some thermoset plasticmolecules are positioned in more than one component thermoset plasticsheet.
 2. The composite thermoset plastic sheet of claim 1 where theseam does not include a separate adhesive.
 3. The composite thermosetplastic sheet of claim 1 wherein at least one of the component thermosetplastic sheets comprises a polyimide.
 4. The composite thermoset plasticsheet of claim 1 wherein at least one of the component thermoset plasticsheets includes a coating.
 5. The composite thermoset plastic sheet ofclaim 1 wherein the plurality of component thermoset plastic sheetsfurther comprises at least a first component thermoset plastic sheet anda second component thermoset plastic sheet, wherein the first componentthermoset plastic sheet is of a different material than the secondcomponent thermoset plastic sheet.
 6. The composite thermoset plasticsheet of claim 1 wherein at least one component thermoset plastic sheetincludes a coating, and the coating comprises polytetrafluoroethylene(PTFE).
 7. The composite thermoset plastic sheet of claim 1 wherein thecomponent thermoset plastic sheets are comprised of a polymer selectedfrom the group consisting of polyimides, polybenzoxazoles, polyimidefilled polytetrafluoroethylene, and any combination thereof.
 8. Acomposite plastic sheet comprising: a plurality of component plasticsheets; a seam, wherein at least two component plastic sheets are indirect contact in the seam; and a diffusion weld connecting at least twocomponent plastic sheets in the seam where at least one of the connectedcomponent plastic sheets is a thermoset plastic.
 9. The compositeplastic sheet of claim 8 wherein at least one component plastic sheetincludes a coating.
 10. The composite plastic sheet of claim 8 whereinthe component plastic sheets include at least two different types ofplastic sheets.
 11. A composite plastic sheet comprising: a plurality ofpolymeric sheets including at least a first polymeric sheet and a secondpolymeric sheet; a seam, wherein at least the first polymeric sheet andthe second polymeric sheet are in direct contact in the seam; and aplurality of spot diffusion welds connecting the plurality of polymericsheets in the seam.
 12. The composite plastic sheet of claim 11 where atleast one polymeric sheet is thermoset.
 13. The composite plastic sheetof claim 11 where each spot diffusion weld is isolated from the otherspot diffusion welds.
 14. A method of seaming polymeric sheetscomprising: (a) providing a plurality of polymeric sheets, wherein atleast one polymeric sheet is a thermoset polymeric sheet; (b)overlapping the polymeric sheets to produce an interface such that thepolymeric sheets are in direct contact at the interface; and (c)applying heat and pressure to the interface to diffusion weld thepolymeric sheets.
 15. The method of claim 14 wherein at least onepolymeric sheet includes a coating and a core.
 16. The method of claim15 wherein the coating is a metallic material, the method furthercomprising: removing the coating from a portion of the core such thatthere is no coating between the polymeric sheets at the interface beforestep (b).
 17. The method of claim 15 wherein step (b) further comprisespositioning the polymeric sheets such that the coating directly contactsanother polymeric sheet at the interface.
 18. The method of claim 15wherein the core comprises a thermoset material.
 19. The method of claim15 wherein the coating comprises polytetrafluoroethylene (PTFE).
 20. Themethod of claim 14 further comprising tensioning the polymeric sheetsbefore step (c).
 21. The method of claim 14 wherein the plurality ofpolymeric sheets includes at least two polymeric sheets comprised ofdifferent materials.
 22. The method of claim 14 wherein the plurality ofpolymeric sheets are thermoset plastics.
 23. A method for seaming apolymeric material, comprising: (a) providing at least two polymericsheets including at least one edge, a first side and a second side,where the first side comprises a metallic coating; (b) providing apolymeric connecting strip; (c) aligning the polymeric sheets such thatthe sheet edges abut each other; (d) covering the abutting edges withthe polymeric connecting strip to form an overlapped interface, wherethe polymeric connecting strip directly contacts the second side of eachpolymeric sheet; and (e) applying heat and pressure to the polymericconnecting strip to diffusion weld the polymeric sheets to the polymericconnecting strip.
 24. The method of claim 23 where the polymeric sheetsand the connecting strip comprise a thermoset polymer.
 25. A method forseaming polymeric sheets comprising: (a) providing a plurality ofpolymeric sheets; (b) overlapping the polymeric sheets to produce aninterface such that the polymeric sheets are in direct contact at theinterface; and (c) applying heat and pressure to the interface atdiscrete spots to form a plurality of spot diffusion welds in theinterface.
 26. The method of claim 25 where at least one polymeric sheetis thermoset.
 27. The method of claim 25 where at least one polymericsheet comprises a polyimide polymer made from pyromellitic dianhydrideand oxydianiline (PMDA/ODA).